Apparatus for the separation of the gases of air by cryogenic distillation use most often the conventional double column arrangement. The liquids produced at the bottom (liquid rich in oxygen), in the intermediate part (lean liquid) and at the top (liquid nitrogen) of the medium pressure column (or MP column), are sent to an intermediate point or to the top of the low pressure column (or LP column). Most often, for economic reasons, the low pressure column is placed above the medium pressure column. It is thus necessary to convey the liquids to a point located higher than the point from which they are removed. Conventionally, the pressure difference between the medium pressure column and the low pressure column is greater than the hydrostatic pressure of the liquid column between the removal point from the MP column and the inlet point of the LP column.
The recent evolution of the technology of air distillation columns has seen the appearance, on the one hand, of packed columns with low pressure drop, and on the other hand, of vaporizer/condensers with small temperature difference between the two fluids in heat exchange relation (gaseous nitrogen and liquid oxygen). These two improvements permit a reduction of the operating pressure of the medium pressure column, so as to reduce the expenditure of energy, and also an increase in the height of the columns, the economic optimum being in the direction of a more extended distillation.
On the other hand, an unfavorable consequence of this development is the difficulty of raising the liquids by simple hydrostatic effect. Thus, under certain conditions, the pressure available in the MP column is not sufficient to raise the liquids to the LP column, particularly when the apparatus must be subject to reduced operation, which is to say with a lower pressure of the MP column than under rated conditions.
The state of the art permits solving this problem using pumps which raise the liquids to sufficient pressures. The drawbacks are obvious: cost of energy, cost of investment, reduced reliability of the apparatus, greater complexity of operation, etc.